1. Field of the Invention
The invention relates generally to the field of vibrators used as sources of seismic energy for surveying the Earth's subsurface. More particularly, the invention relates to vibrators that use the superheterodyne principle to produce very low frequency seismic energy.
2. Background Art
Vibrator-type seismic energy sources known in the art impart seismic energy into the Earth's subsurface by moving a radiating surface in a particular manner. See, for example, U.S. Pat. No. 3,863,202 issued to Landrum, Jr. In one type of implementation, the radiating surface is coupled to a control system including a hydraulic ram and a control valve that selectively applies hydraulic pressure to each side of the hydraulic ram. The control valve is typically electrically operated. Electrical signals applied to the control valve generally correspond to the vibratory waveform that it is intended to be produced by the motion of the radiating surface. In order for the motion of the radiating surface to be efficiently coupled to the Earth, it is necessary to provide a large reactive mass coupled to the hydraulic ram opposite the radiating surface. In another implementation, a diaphragm placed in a body of water is moved in a similar manner through either electrical or electro-mechanical means.
A particular limitation to vibrator seismic sources known in the art relates to generating low frequency seismic energy, typically less than about 8 Hz. For such low frequencies, the reactive mass or diaphragm must be relatively large, and the amount of motion that must be imparted to the radiating surface is also relatively large. Controlling such motion so that it faithfully corresponds to the electrical control signal has also proven to be difficult. Additionally, vibrators known in the art are limited with respect to the highest frequencies they can accurately generate because of coupling difficulties between the driver and the acoustic radiating the surface and limitations in the speed at which hydraulic mechanism can react. Typically, conventional vibrators cannot operate effectively above about 150 Hz. There continues to be a need for improved vibrator-type seismic energy sources for subsurface seismic surveying.